A: Electrophotographic Photoreceptor
Electrophotographic photoreceptors which are sensitive to the visible light range have been developed for the purpose of application to copiers, photoprinters and the like. As such electrophotographic photoreceptors there have heretofore been widely used photoreceptors comprising as the main component an inorganic photoconducting substance such as selenium, zinc oxide and cadmium sulfide. However, such an inorganic photoreceptor does not necessarily satisfy all requirements for electrophotographic photoreceptors for copiers or the like, e.g., light sensitivity, thermal stability, moisture resistance and durability.
For example, selenium photoreceptors are subject to crystallization due to heat or stain such as fingerprint caused by touch of the hand and hence there is deterioration in the above-mentioned properties required for electrophotographic photoreceptors.
Electrophotographic photoreceptors comprising cadmium sulfide exhibit poor moisture resistance and durability. Electrophotographic photoreceptors comprising zinc oxide leave to be desired in durability such as film strength. Further, due to their toxicity, selenium and cadmium sulfide have a great restriction in production and handling.
In recent years, in order to eliminate these disadvantages of inorganic photoreceptors, photoreceptors comprising various organic substances have been developed, and some of them have been put into practical use. Examples of these organic photoreceptors include an electrophotographic photoreceptor comprising poly-N-vinyl carbazole and 2,4,7-trinitrofluorenone-9-one (U.S. Pat. No. 3,484,237), an electrophotographic photoreceptor obtained by sensitizing poly-N-vinyl carbazole with a pyrilium salt dye (JP-B-48-25658 (the term "JP-B" as used herein means an "examined Japanese patent publication"), an electrophotographic photoreceptor comprising an organic pigment as main component (JP-A-47-37543 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and an electrophotographic photoreceptor comprising as main components a eutectic complex of a dye and a resin (JP-A-47-107785).
Although these photoreceptors give some improvements in the above-mentioned inorganic photoreceptors, they generally exhibit a low light sensitivity and are not suited for repeated use. Thus, these photoreceptors leave to be desired as electrophotographic photoreceptors.
In order to eliminate such a disadvantage, an electrophotographic photoreceptor has been proposed comprising a separate function type photoconducting layer wherein a charge-generating effect and a charge-transporting effect are separately accomplished by different substances. This kind of an electrophotographic photoreceptor has become the main subject of study. Such a separate function type electrophotographic photoreceptor is advantageous in that the range of materials to be incorporated therein can be extended, making it possible to improve the sensitivity and durability thereof. This kind of an electrophotographic photoreceptor is also advantageous in that selection can be made from a wide range of materials favorable to the formation of coating materials for forming electrophotographic photoreceptors.
As effective organic charge-generating materials to be incorporated in the charge-generating layer in such a separate function type electrophotographic photoreceptor there have been developed various organic dyes and organic pigments. For example, azo pigments, perylene pigments, polycyclic quinone pigments and squaric methine dyes having various structures have been used.
These pigments exhibit a relatively high sensitivity in a short wavelength range or middle wavelength range but exhibit a low sensitivity in a long wavelength range. Thus, these pigments can be hardly used for laser printers using a highly reliability semiconductor laser. Gallium-aluminum-arsenic light-emitting elements which are widely used as semiconductor lasers at present have an oscillation wavelength of 750 nm or more.
A phthalocyanine compound, which is one of the organic photoconducting materials, has been known to have a sensitivity range extended to the long wavelength side as compared to the above-mentioned pigments and dyes but exhibit insufficient electrophotographic properties such as sensitivity and chargeability. In order to overcome these difficulties, studies have been made to develop phthalocyanine compounds with various center metals or various crystal forms. Various crystal forms of phthalocyanine compounds have been found in the course during which an unstable .alpha.-type phthalocyanine is converted to a .beta.-type phthalocyanine having a crystalline stability. For example, .epsilon.-type copper phthalocyanine, X-type metal-free phthalocyanine, and m-type titanyl phthalocyanine have been known. These phthalocyanines are sensitive to a long wavelength range but exhibit too low a sensitivity to be used for copiers or photoprinters. These phthalocyanines are also disadvantageous in that they lack potential stability or exhibit a large residual potential upon repeated use. Thus, these phthalocyanines cannot be put into practical use.
On the other hand, in an attempt to improve the sensitivity of an electrophotographic photoreceptor comprising a phthalocyanine pigment, the addition of a charge-transporting compound such as a hydrazone compound or an oxazole compound or an electron-attracting compound such as tetranitrofluorenone and trinitrofluorenone has been proposed. These approaches were found to give an effect of sensitization. However, the sensitizing effect thus attained is insufficient. These approaches are also disadvantageous in that the addition of these additives causes a drop in chargeability or a drop in potential stability or sensitivity or a rise in residual potential upon repeated use. Thus, none of these approaches could be put into practical use. Further, due to toxicity, the above-mentioned electron-attracting compounds cannot be put into practical use.
As has been described, an electrophotographic photoreceptor has been desired which is highly sensitive, particularly to light of a long wavelength of 750 nm, and exhibits a high potential stability, a small residual potential and a small drop in sensitivity upon repeated use.